This invention relates generally to compression molding of polymers and composites by use of microwave energy, and more specifically to the design of molds for processing of such materials including the selection and/or modification of mold materials to form the mold to provide for uniform heating of polymers and composites by microwave energy. The molds and processes disclosed herein are adapted for molding parts of uniform thickness, variable thickness or complex shape. The developed technique may be used for molding of wide variety of polymers and composites from zero loss factor to conductive grades not withstanding the dielectric or thermal properties of the polymers.
Modern high performance engineering polymers known under trademark names PEEK®, TORLON®, SEMITRON®, DURATRON®, CELAZOLE®, and others perform at extremely high temperatures, well above 500° F., with superior mechanical and chemical properties. Compression molding of such polymers to stock and custom shapes by microwave energy offers a promising alternative to conventional compression molding techniques generally utilizing electric, gas or steam heating to heat the polymer or work material. For this reason, designing a mold, capable of processing a variety of such polymers by microwave energy, is of great practical importance.
As used herein, the polymer, composite, or ceramic materials to be processed in the mold cavity may be referred to as the work material or material to be molded and the part or component to be formed thereby may be referred to as the workpiece or molded part. The work material may be supplied in pellet, powder, liquid or solid form. Although injection molding is widespread, processes for preheating polymer powders and pellets or granules are still necessary because for certain end products, compression molding is preferred. For injection molded parts the thickness of the part to be molded is limited by the relationship of the flow length versus thickness of the workpiece. For this reason relatively thick parts of some polymers must be compression-molded.
The process of heating polymer powders with conventional techniques is very slow due to poor thermal conductivity of the polymer. When using only thermal conductivity to heat the work material, heat flows from the polymer surface toward interior regions, which therefore necessitates an extended period of heating time to equalize the temperatures through the entire volume of the work material without overheating of its surface. The heating time of conventional heating means may exceed several hours depending on the thickness of the part to be molded. Increased heating time may require the use of grain growth inhibitors, which usually reduces mechanical strength of the polymer. It also boosts energy consumption and the per unit parts cost.
In conventional compression molding processes of polymers, high temperatures, long processing times, and, in some cases, hot pressing must be applied in the fabrication of products to achieve the highest density and minimum porosity. Conventional compression molding of polymers involves the compaction of a polymer powder into the desired shape following by sintering. The powder is placed in a mold and compacted by applying pressure to the mold halves. The compacted powder is usually porous and its porosity depends upon the amount of applied pressure and the resistance of the particles to deformation. The compacted powder is then heated in the conventional oven to promote bonding of the powder particles. The sintering temperature causes diffusion and neck formation between the powder particles resulting in a dense body.
The uniform heating and molding of polymers in microwave ovens has unique advantages over conventional compression molding. The use of microwave energy reduces processing time by a factor of 10 or more. The shortened process time minimizes grain growth. A fine initial microstructure retains the same grain size without using grain growth inhibitors and allows achievement of a high mechanical strength. It is believed that the disclosed microwave process will produce products having improved mechanical properties with additional benefits of short processing time and significantly reduced energy usage along with clean environment.
Compression molding of high performance engineering polymers often requires compression at temperatures above 700° F. and pressures around 2000 psi for prolonged periods of time to provide high quality consolidation of the powdered or granular work material. For example, Polybenzimidazole, known under trade-mark name CELAZOLE®, has one of the highest heat deflection temperatures of 800° F. at 264 psi and must be processed at temperatures well above 800° F. and at high pressures. The ceramic molds developed and disclosed in my above mentioned U.S. patent applications may be susceptible to chipping and wearing after long runs in such extreme environments. The employed metal reinforcing rings extending around the side mold wall, significantly strengthen the ceramic mold in the radial direction but cannot provide the same strength in the longitudinal direction, which may shorten the life of mold. By increasing the number of reinforcing rings, the desired mold strength may be achieved in the radial direction but not in the longitudinal direction.
Although it might seem logical to replace the reinforcement rings by one solid metal tube extending around the ceramic sidewall of the mold to increase the strength of the mold in both radial and longitudinal directions, such a design has drawbacks. The primary drawback being that the metal tube surrounding the ceramic sidewall will act as a shield preventing uniform heating of the sidewall of the mold. This will result in significant non-uniform heating of the work material, making it impossible to achieve uniform compaction of polymer powder or granules. Another drawback is chipping and wearing of inner ceramic surface after long run of the mold.
The known prior art does disclose the use of molds adapted for microwave heating that are made partially or completely of metal or other electrically conductive materials. For example, in U.S. Pat. No. 5,202,541, a plurality of ceramic articles or work pieces to be sintered, are buried within a powder bed made of either microwave transparent material or high loss material. This work piece assembly is surrounded by a crucible which may be made either from microwave transparent material or microwave suspector material for additional heating of work assembly by conduction heating. Such a crucible contains thin metal rings for fixing the field around work piece assembly, but which are not designed for reinforcement of assembly. U.S. Pat. No. 5,202,541 teaches against positioning the rings too close together because doing so will result in non-uniform heating as discussed above. The preferred spacing between rings is identified as 10–20 mm at a frequency of 2.45 GHz.
U.S. Pat. No. 4,617,439 discloses a process for vulcanizing and polymerizing a work material in the form of relatively thin sheets placed between metal plates. The sandwich structure is then pressed and placed in the resonance cavity of a microwave oven for heating by microwave energy. Metal plates are used for compression and shaping of work material and also to provide uniform heating. The drawback of this apparatus is that it cannot be used for processing relatively thick parts from powders or liquids since there is no sidewall.
U.S. Pat. No. 4,323,745 discloses a mold for uniform heating of a work material having a relatively high loss factor. The mold is made of thick ceramic material transparent to microwave energy and has a cavity for placement of the work material. The ceramic enclosure is contiguously enclosed by a metal enclosure having enough mechanical strength to withstand process pressures. Microwave energy is introduced into the inner cavity of the metal enclosure via waveguides or coaxial lines from two generators operating at slightly different frequencies. The metal enclosure serves as both the mold and the microwave resonance cavity.
A similar mold concept can be found in U.S. Pat. No. 5,844,217, which discloses an apparatus for molding liquid thermoset resins by microwave energy. The method uses a metal mold having inner cavity where the work material may be placed and processed. Microwave energy is introduced into a part-shaped mold cavity via multiple ports.
Another mold design utilizing a similar concept may be found in U.S. Pat. No. 4,269,581, where thermoset resin is cured in the space formed by coaxial metal conductors of a coaxial transmission line. The main disadvantage of the apparatus disclosed in the above noted U.S. Pat. Nos. 4,323,745, 5,844,217, and 4,269,581 is that the resonance cavities are part-shaped and require modification with changes in the shape and dimension of the work piece to be molded. This is very costly and time consuming.
A more economical approach is the concept of a batch microwave system having a metal chamber as applicator and means for introducing of microwave energy into this chamber. Different molds of different dimensions and shapes can be placed inside this chamber for processing of work pieces of variable shapes. In contrast with dual function of mold cavity (mold cavity+resonance cavity), the separation of the mold from the resonance cavity allows the system to be more flexible and efficient.
It is known in the food industry and for sterilization purposes to use a metal container covered with a layer of microwave absorbing material to heat different objects positioned in the container. U.S. Pat. No. 5,258,596 discloses a thin walled container formed from a microwavable foil having a layer of organic material containing microwave absorbing dielectric and magnetic components located on the outside surface of a thin metal layer. Such a container formed from a thin metal layer with a thin microwave absorbing coating and with an item to be heated positioned inside may be placed into the chamber of a microwave oven to heat the item. In this case, the heat is developed initially in the coating due to its exposure to the microwave field. Then, the heat is transferred through the metal layer and finally to the item to be heated by thermal conduction and radiation. In a disclosed embodiment the microwave absorbing layer has a thickness of approximately 3 mil (3 thousandths of an inch) and the metal containing layer has a thickness of approximately 21 mil. However due to the thin walled nature of such containers, they are not appropriate for use in forming a compression mold. In addition, such a thin walled container cannot provide effective heating for a relatively large mold or item to be molded.
There remains a need in mold design for microwave molding of polymers, which can withstand high pressures and high temperatures and provide uniform heating by using batch microwave system.